The invention relates to the general field of electrostatic devices with particular reference to the prevention of charge accumulation during attachment/detachment cycles.
Over the past several years, much progress has been made in the development of electrostatic actuators mainly through adoption of the same photolithographic and etching techniques as those employed for fabricating integrated circuits. Micro electromechanical systems (MEMS) devices employ small air gaps (below the Paschen minimum) so can sustain relatively large electric fields without breakdown. The electrostatic forces produced by these large fields are comparable to counteracting mechanical forces present in devices of this size, thereby making electrostatic control of motion and position feasible.
In FIG. 1 we show, in schematic representation, an example of an electrostatic actuator based motor known as a wobble motor. The principal components are a rotor 12. made of an insulating material, and a stator in the form of a tube inside which the rotor 12 rotates. This stator is, essentially, a series of electrodes 13 (the actuators) that are insulated from one another and from the rotor by being encased within dielectric material 14. Voltage is successively applied to succeeding electrodes 13 in a constant direction, such as 10, so that rotor 12 is constantly being attracted to the next actuator in line, resulting in its rotating in direction 11.
FIG. 2 is an example of a linear motor having multiple stators such as 26. Each such stator has the form of a rigid beam made up of (in this example) four actuators such as 27 or 28 encased in a dielectric 14. The rotor 21 in this design is a beam of insulating, as well as flexible, material of a similar size and shape as the stator. Depending on which actuators are charged, the shape of the rotor can be altered. In FIG. 2, uncharged actuators (such as 27) are shown as hatched while charged actuators (such as 28) are shown as solid. Three possible shapes, 22, 23, and 24, for the rotors are shown in this example. By cyclically adjusting the rotor shapes, rotary (via crankshaft) or linear (reciprocating) motion can be achieved.
Structures such as those just discussed, that utilize electrostatic forces to attract members into direct contact with one another, are known to suffer from charging effects whereby charge gradually builds up on the surface of the dielectric 14. This charging behavior manifests itself in two ways: as a variation in the potential required to attract the members into contact and as a time delay in the separation of the members after the applied potential has been removed.
The problem has been classified as the attachment /detachment cycle and has been reported in a number of papers. For example, K. M. Anderson and J. E. Colgate in xe2x80x9cA model of the attachment/detachment cycle of electrostatic micro actuatorsxe2x80x9d, DSC Vol 32, Microelectromechanical Sensors, Actuators, and Systems, ASME 1991 pp 255-268, have identified several mechanisms which provide valid explanations for the observations. These include ionization in the air gap, tribo charging due to dissimilar materials, and surface micro conduction.
Ionization has been confirmed as a mechanism by Wibbler et al. (wibbeler J, Gunter P, and Hietschold M, xe2x80x9cParasitic charging of dielectric surfaces in Capacitive micro electromechanical systems (MEMS)xe2x80x9d, Sensors and Actuators A71 (1998) pp 74-80).
Tribo charging is commonly eliminated by ensuring that the materials that are brought into contact with one another have similar work functions.
Surface microconduction can be minimized by reducing the contact area but this solution does not eliminate the problem.
Of interest, also, is work on the charging problem that was done in two fields unrelated to electrostatic microstructure, but using materials to which the present invention could be applied. These are xe2x80x9cThe design of quasi electrostatic induction actuatorsxe2x80x9d by Egawa S, Niino, T, and Higuchi T, in xe2x80x9cFilm actuators: planar electrostatic surface-drive actuatorsxe2x80x9d, (IEEE CH2957-9/91 pp. 9-14) and Egawa, S and Higuchi, T in xe2x80x9cMulti-layered electrostatic film actuatorxe2x80x9d, (CH2832-4/90 pp 166-171). In the latter, low mobility of charge in high resistivity materials is exploited as a mechanism for generating force in a moving electrostatic field.
In the fabrication of high voltage integrated circuits, semi-insulating polysilicon (SIPOS) has been used as a means of passivating electronic structures and limiting the surface charge sensitivity and external electric field sensitivity of devices by providing a resistive field shield. For example, Jaume D, Charitat G, Reynes J M, and Rossel P in xe2x80x9cHigh-voltage planar devices using field plate and semi-resistive layersxe2x80x9d, (IEEE Transactions on Electron Devices Vol 38 No. 7 July 1991 pp 1681-1684); Held R, Serafin J, Fullmann M, Constapel R, and Korec J, in xe2x80x9cInvestigation of static and dynamic characteristics of SOI-LDMOSFETs passivated with semi-insulating layersxe2x80x9d, (5th International Symposium on Power Semiconductor devices and IC""s 1993 pp 130-134); and Charitat G, Jaume D, Peyre-Lavigne A, and Rossel P, in xe2x80x9c1000 and 1500 volts planar devices using field plate and semi-resistive layers: design and fabricationxe2x80x9d, (IEDM 90-803 pp. 32.5.1-32.5.4.
A routine search of the patented prior art was conducted but no references that teach the same solution to the charging problem as that disclosed in the present invention were uncovered. Of interest is a description by Cabuz et al. in U.S. Pat. 5,822,170 of a hydrophobic coating for reducing humidity effects in electrostatic actuators This addresses the effects of charging due to moisture related effects by a hydrophobic surface treatment.
It has been an object of the present invention to provide an electrostatically actuated device which is not subject to charging effects that either slow down the operation of the device, gradually increase the voltage required to operate the device, or cause parts of the device to adhere to each other at the wrong time.
Another object of the invention has been to provide a method for forming said electrostatic device.
A further object has been that said method not change significantly the operating characteristics of the device.
These objects have been achieved by replacing the dielectric material that is normally present between the force generating conductor surfaces with a semi-insulating material. This semi-insulating film overcomes the effects of charging, while avoiding short-circuits when the surfaces are pulled into contact. It is not subject to insulation breakdown within the range of voltages used to operate the device.